1. Field of the Invention
The present invention relates to an image forming apparatus and a control method of the apparatus.
2. Description of the Related Art
An apparatus having a print function, such as printers, copying machines or the like, interprets print data described in a page description language, converts rendering commands in the print data to intermediate data which is suitable for rendering, and stores the intermediate data in a memory. In accordance with the intermediate data stored in the memory, the apparatus performs processing for bitmap data rendering in a rendering memory. When the apparatus interprets print data and stores intermediate data in a memory, there is a case that the memory becomes full (memory overflow). In this case, the print data interpretation and intermediate data generation are temporarily suspended, and rendering is performed using the intermediate data that has already been stored in the memory. Then, the rendered image is registered in another memory, which has been prepared in advance, as a background image of intermediate data that will subsequently be generated. This process is called fallback processing. In the fallback processing, the background image is encoded for compression in order to vacate the memory area for the next intermediate data. If lossy-encoding is adopted in the encoding process, image quality deteriorates. Furthermore, if the background image encoded by an initial lossy-encoding does not fit in a compressed image data storage area, image rendering must be performed again based on the intermediate data and re-compression must be performed with a different quantization coefficient.
In a case where the background image cannot fit in a compressed image data storage area, conventionally, a technique is available in that the compression rate of image data included in the intermediate data is increased (e.g., Japanese Patent Laid-Open No. 2002-127510).
JPEG XR introduced in “Current report of standardization for the JPEG XR image compression”, The Journal of The Institute of Image Electronics Engineers of Japan, Vol. 37, No. 4, pp. 502-512, issued Jul. 25, 2008 or in Nikkei Electronics issued on Dec. 29, 2008, p71 to 77 (issued on Dec. 29, 2008) bases HD Photo, the still-image encoding technique. JPEG XR is characterized by breaking down an image to be encoded into tile areas of variable sizes. For instance, the central part of an image is broken down to small areas while the edge part of the image is broken down to large elongated areas, so as to enable an efficient access to the central part of the image, which is assumed to be the area of interest.
In fallback processing, if lossy-encoding is employed for compressing the entire rendered background image, image quality may deteriorate. Furthermore, if lossy-encoding is employed on image data of intermediate codes as in the aforementioned prior art, rendering must be performed again. Moreover, since plural numbers of times of image encoding and decoding may be necessary, print performance reduction is caused.